why am i getting platform not supported error
This has been an issue since the airship map was added to the game. The itch.io PC version of Among Us is no longer supported by the devs. If you aren't affected by the problems some of us have with Steam (like it being a known health risk since late 2019 with valve openly refusing to fix it), you can use the Steam key which is available on the download page. If you don't want to use Steam for any reason, you should contact the devs and if necessary, approach itch.io for a refund.
The devs have said, after I got my refund already, that they're looking into possibly resuming support for this version of the game, but haven't provided any kind of timeframe on doing so, or any update which makes it work correctly.
You can also play the game through itch.io as a "guest" instead of creating an account but that locks out a bunch of in-game functionality including the ability to customise your name and use the proper chat function.